When verify fails, dump the good/bad blocks to files

This makes it easy to compare afterwards to see what kind of
corruption was experienced.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/ioengine.h b/ioengine.h
index 48a042b..c56bd50 100644
--- a/ioengine.h
+++ b/ioengine.h
@@ -49,6 +49,11 @@
 	unsigned long long offset;
 
 	/*
+	 * Initial seed for generating the buffer contents
+	 */
+	unsigned long rand_seed;
+
+	/*
 	 * IO engine state, may be different from above when we get
 	 * partial transfers / residual data counts
 	 */